Mass spectrometry



June 11, 1957 c. F. ROBINSON MASS SPECTROMETRY 2 Sheets-Sheet 1 FiledMay 18, 1955 IN V EN TOR.

C HARLE 3 f. ROB/NSDN M, Q A77OANEYS MASS SPECTROMETRY Application May18, 1955, Serial No. 509,141

5 Claims. (Cl. 250-41-9) This invention is concerned with massspectrometers. It provides improvements for protecting an ion beam to beanalyzed from the effects of static charges built up within thespectrometer so that the calibration remains unchanged over longerperiods and the accuracy of analysis is improved.

A mass spectrometer is essentially an apparatus for producing ions andsorting them according to the ratio of their mass to their charge, i. e.according to their spe cific mass. A sample to be analyzed, such as agas mixture, is ionized, usually by electron bombardment, and theresulting ions are propelled by an electrical potential into ananalyzer. The ions enter the analyzer as an unsorted beam, and in theanalyzer are sorted under the influence of an electromagnetic orelectrical field, or both, into a series of divergent homogeneous beamsof ions having the same specific mass. The sorted ions are collected anddischarged, the quantity of each kind of ions being measured by theamount of current that they discharge upon collection.

In some types of mass spectrometer the ions have an energy in the orderof volts so that any static charges built up on the inner surfaces ofthe analyzer chamber will seriously impair resolution of the instrumentif focusing makes use of an electrical field.

In the type of mass spectrometer to which this invention is directedprimarily, ionization of the molecules is accomplished in an ionizationchamber, or ion source, by bombardment with an electron beam. Theresulting ions diffuse or are propelled into the analyzer chamber andare sorted under the influence of crossed magnetic and electric fields,which cause the ion beams to travel in characteristic cycloidal paths.Such a path is that traced by a point on a radius of a circle rolling ina plane along a line in the plane, which in this instance is a planeperpendicular to the magnetic field, and may also 1 e considered asbeing rigorously circular in a coordinate :system moving with uniformvelocity. The velocity of the coordinate system or the center of therolling circle is solely a function of the ratio of the electric andmagnetic field strengths.

From the periodic property of the cycloid a charged particle which isintroduced into transverse magnetic and electrical fields and whichcrosses a plane normal to the electric field will again cross this sameplane when the particle is going in the same direction of travel at adistance proportional to its mass-to-cha-rge ratio. And if theelectrical field strength is uniform so that the coordinate systems orcenters of the rolling circles corresponding to each ion move at thesame velocity, ions of a particular mass will converge to a series ofidentical focal points after any Whole number of turns in the magneticfield regardless of their velocity or direction of travel at the momentof introduction into the field.

In a typical instrument of this type the electrical field is establishedin the space defined by the analyzer body by a plurality of spaced,parallel electrodes, with one of the electrodes defining a so-calledfocal plane and two 2,795,701 Patented June 11, 1957 ice 21rmEc X Qwhere B=the magnetic field strength in Gauss;

E: the electrical field strength in statvolt/cm.; q: the ion charge ine. s. u.;

m=the ion mass in grams; and

c=the speed of light in cm./sec.

Since in practice the focal distance X1 is fixed, the ion mass m isdetermined by calculation from ascertained values of the electric fieldstrength E and the magnetic field strength B. It is thus of utmostimportance that the electric field strength be accurately ascertainedand that uniformity be maintained to the greatest extent possible.

However, it has been found that some of the ions formed in a cross-fieldmass spectrometer tend to lodge upon the electrodes used to produce theelectric field in the mass spectrometer and to build up static chargeson the surfaces of these electrodes. When an ion beam strikes a surfaceat near-grazing incidence, the surface tends to acquire a localizedcharge such that the electric potential of the surface is different fromthe potential of the underlying metal. The surface potential may thus bedifferent from the potential of the underlying metal by as much as 70volts.

In a conventional mass spectrometer the ion beam does not strike asurface at near-grazing incidence except when the beam has a verysubstantial amount of energy, of the order of several hundred or a fewthousand volts, and such surface charges have a comparatively smalleffect if good cleaning methods are used on the surfaces. However, ininstruments such as cycloidal mass spectrometers in which the dependenceof focal distance on the ion energy is reduced or eliminated, anelectrical field must be provided, usually by a plurality of electrodes.And since two of such electrodes in practice form the opposite innersurfaces of the analyzer chamber body, it is inherent that if therequired electrical field is obtained by electrodes heretofore existingin the art, grazing incidence of ions will occur and the resolution willbe impaired seriously by the resultant build-up of surface charges,causing nonuniformity in the electric field and creating potentials ofunknown values.

The first order effect of nonunifonrnity in the electrical field is notto destroy the quality of the focus but to displace the position inwhich this focus occurs, compared to an instrument in which a collectorelectrode is located at the proper point of focus based uponconsideration of uniform electrical and magnetic fields. In the eventthe point of focus is displaced as a result of any nonuniformity of theelectrical field, resolution is impaired since the focal point no longercoincides with the resolving slit.

The magnitude of the effect can be much reduced by careful cleaningprocedures. But even if the cleaning has been thorough at the outset,the surfaces will become contaminated through use and the effect willmake its appearance in a short-time which may be from a. few hours to afew weeks depending on the initial condition of the surface. It is alsopossible to alleviate the elfects of surface charges or polarization byusing a very large electrode structure so that the surfaces whichexhibit polarization are far removed from the ion beam of interest. Thiscauses the unit to become much bulkier and more expensive thannecessary. 1

I have now developed a means which mitigates the effects of surfacecharges on the field electrodes, and which possesses distinct advantagesover previously suggested methods. This objective can be accomplished byarranging the unit so that a given field electrode appears to define aplane as far as the electrical field configuration is concerned.However, that part of the electrode in which grazing incidence of ionswill occur is actually a series of louvers such as the slats of aVenetian blind. The louvers are inclined so that an ion striking theelectrode will strike a louver surface facing away from the ion beam andthus the ion beam of interest will be shielded fromqthe resultingsurface charge. Such an arrangement alleviates the effects of surfacecharge by permitting them to build up only in locations where they do noharm. Moreover, since the louvers may be constructed so that the spacebetween them is opened through the thickness of the electrode, theelectrode structure which I have developed has the additional advantageof increasing pumping speed which further reduces the tendency forsurface deposits to form.

V The invention will be more clearly understood by reference to theaccompanying drawings, in which:

Fig. 1 is a schematic sectional elevation of a cycloidal massspectrometer in accordance with a particular adaptation of theinvention;

Fig. 2 is a schematic sectional elevation taken along lines 22 of Fig. 1in enlarged scale showing the plan view of the louvers in theelectrodes;

Fig. 3 is a schematic sectional elevation of a conventional cross-fieldmass spectrometer showing the effect of static charges upon the ionbeam; and

Fig. 4 is a schematic diagram showing the electrical circuitry of themass spectrometer of Fig. l.

The mass spectrometer shown in Fig. 1 comprises an envelope having theusual connection 11 to a vacuum system (not shown) and sample inlet line12. A series of electrodes 13, 14, 15, 16, 17, 18 are supported in theenvelope in parallel, uniformly spaced relation. Electrode 16 defines aso-called focal plane and is provided with an ion inlet aperture 19 anda resolving aperture 20 Additionally, electrode 16 has relatively largeopenings 21, 22 to permit ion travel in a cycloidal trajectory.

Electrodes 13, 14, 15, 16, 17 and 18 are connected to a voltage divider23 across which a voltage is applied from a variable source 24. By meansof the electrodes and the divider network illustrated schematically, auniform electrical field is applied across the region defined by theelectrodes. The envelope is subjected to a magnetic field developed bymagnet poles, one of which is shown as the circular pole 25. Thedirection of the magnetic field is normal to the uniform electricalfield.

An ion source 26 is disposed adjacent the ion entry slit aperture 19 inelectrode 16. Shown schematically in Fig. 4, the ion source is inconventional form, including a repeller electrode 27, an acceleratingelectrode 28, an electron gun 29 and an electron target 30. By means ofthe electron gun and target an ionizing electron beam is directed acrossthe ion source. The repeller electrode 27 .and accelerating electrode 28are connected to a voltage divider network 31 which is in turn connectedacross the voltage source 24. Electron gun 29 is connected through atransformer 32 to an A. C. source 33 for filament heat and through abias battery 34 to the ac celerator 28. The target 30 is connectedthrough a bias battery 35 to the accelerator 28.

A sample to be analyzed is introduced through inlet 12 which is directlyconnected into the ion source 26. Under influence of the ionizingelectron beam the samconnector 11 to vacuum pumps not shown.

plc is ionized. Ions from the source pass through, the inlet aperture 19and into the analyzer chamber 42 by the action of an acceleratingpotential existing between the repeller electrode 27, acceleratingelectrode 28 and plate electrode 16.

In an aperture in the middle portion of the electrode 13 thin louvers 38are placed and conductively attached to electrode 13. The louvers asshown are parallel, uniformly spaced and inclined into the direction oftravel of ions under the influence of the crossed magnetic and electricfields. The center portion of electrode 18 is likewise provided withlouvers 39 inclined into the path of the ions.

Use of a mass spectrometer constructed in conventional form, as shown inFig. 3, causes static charges to be built up on the inner surfaces ofthe analyzer chamber body when ions, as shown, strike such surfaces atgrazing incidence. Such ions will be heavy ions not in focus or strayions from the focussed beam. Such surface charges create an unknownpotential within the instrument and impair resolution by destroyinguniformity of the electric field.

By contrast, the structure of my invention nullifies such surfacescharges, thereby greatly improving the accuracy and resolution of theinstrument. In the preferred form of my invention which I have heredescribed, the constmction of the electrodes is such that a stray orheavy ion, as shown by the dotted line in Fig. 1, will strike theoutward facing surface of a louver so that any resulting static electriccharge on the surface of the electrode is shielded from the focused ionbeam shown by the solid line in Fig. 1. Thus, surface static chargesbuilt up by stray ions striking an electrode will have little or noefiect on the ion beam of interest.

As will be noted, an additional feature of my invention is that itprovides means for increasing pumping speed. The top cover 40 and thebottom cover 41 which are placed adjacent the louvered field electrodesare provided with conduits 43 and 44 which lead through the The louveredelectrodes provide additional conduits between the analyzer chamber andthe vacuum pumps, thereby increasing pumping speed and the efiiciency ofoperation of the instrument.

I claim:

1. In a crossed-field mass spectrometer, the combination comprising ananalyzer chamber body having an inner surface which defines an analyzerchamber, a source for producing ions connected to the analyzer chamberbody, means propelling 'a beam of ions into the analyzer chamber, meansestablishing 'a magnetic field in a given direction in said analyzerchamber, means establishing an electric field transverse of saidmagnetic field within said analyzer :cham-ber'said electric and magneticfields being adapted to cause said ion beam to travel in a cycloidaltrajectory to arrive at a point where the beam of ions of apredetermined mass is focussed, and louvers affixed to the inner surfaceof the analyzer chamber body to shield the trajectory of the focussedion beam from electrostatic surface charges on'said inner surface.

2. Apparatus in accordance with claim 1 in which the louvers areconductively connected to the analyzer chamber body.

3. In a crossed-field mass spectrometer, the combination comprising ananalyzer chamber body which defines an analyzer chamber, means connectedto the analyzer chamber body for producing ions and propelling ions intotheanalyzer chamber, a plurality of parallel electrodes for establishingan electric field in a given direction within the'body, two of saidelectrodes being adapted to form opposite inner surfaces of the analyzerchamber body, means establishing amagnetic'field transverse of saidelectric field within the analyzer chamber, said electric and magneticfields being adapted to cause a beam of ions of a predetermined mass totravel in a cycloidal trajectory to arrive at a point where the beam isfocussed, and a series of louvers conductively attached to the innersurfaces of said two electrodes to shield the beam "of ions ofpredetermined mass from surface charges on said electrodes caused bygrazing incidence of ions other than those of said predetermined mass.

4. Apparatus in accordance with claim 3 in which the louvers areconductively attached in a parallel spaced relation in apertures in saidtwo electrodes.

5. In a mass spectrometer, the combination comprising an analyzerchamber body which defines an analyzer chamber, a source for producingions connected to the analyzer chamber body, means for propelling an ionbeam into the analyzer chamber, means disposed about the analyzerchamber for imparting a given trajectory within the analyzer chamber toa beam of ions of predetermined mass, and means conductively attached tothe inner surface of the analyzer chamber body to shield said ion beamfrom electrostatic surface charges on sai inner surface.

References Cited in the file of this patent UNITED STATES PATENTS2,221,467 Bleakney Nov. 12, 1940

